Landslides in Vermont · • “The downslope movement of soil, rock and organic materials under the influence of gravity and also the landform that results from such a movement”

Landslides in Vermont

George Springston Norwich University Dept. of Geology and Environmental Science

Northfield, VT, October, 2010

Outline

• Definitions

• Mad River

• Vermont Highway Rockfall Hazards

• Smugglers Notch

• Jeffersonville

• “The downslope movement of soil, rock and organic materials under the influence of gravity and also the landform that results from such a movement” (The Landslide Handbook, USGS Circular 1325, p. 4).

Figure from USGS Fact Sheet 2004-3072 http://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html

Landslide on Winooski River, Cate Farm, Plainfield, 2009

Lori Barg, Spring, 2002

This is prior to the large failure in 2003. Note signs of active slope failure: bare soil, slide blocks, tilted trees, etc).

Spring, 2003

6/3/2003

5/28/2003

Stratigraphy

• 0.7 - 1.2 m pebbly medium sand (fluvial).

• 12.2 m fine sand and silty fine sand with layers of massive silty clay (lacustrine).

• 9.1 m varved silt, silty clay and clay with 2-4 cm couplets (lacustrine).

Fluvial terrace deposit of pebbly medium sand with some cobbles.

Below fluvial deposit is fine sand and silty fine sand showing extensive soft-sediment deformation.

Spring, 2003

Varved clay and silty clay. Two to four cm thick couplets. Winter and summer subequal in thickness.

Since drainage of glacial Lake Winooski, Mad River has cut down about 22 meters.

Dimensions

• 43 m (140 ft) wide • 76 m (248 ft) long (N-

S) • 22 m (70 ft) high • 15.5º slope overall • 4000 m3 (5300 yd3)

volume of displaced mass

Erosion of Toe

• >10 m (33 ft) eroded since 5/28/03

• >700 m3 (900 yards3) carried away by river

• Turbidity visible at least to Waterbury -26 km downstream

5/28/03

1873 Beers Atlas

Washington County

Historical Analysis

1:24,000 Original: USGS WSP 424

1:24,000 Original

No sign of landslide on 1962 air photos

1:20,000 Original

1:30,000 Original

No sign of landslide.

Landslide present.

May 16 May 23

Cumulative Precipitation at USGS stream gage downstream at Moretown.

Sliding reported on 16th and 23rd

Stage (in feet) at USGS stream gage downstream at Moretown.

Apparently no landslide activity right after May 2 peak.

6/3/2003

Triggering the Slide

1. Erosion due to high flows.

2. High ground water levels/soil moisture.

3. Removal of support with falling stage.

T. Eliassen, Vt. Agency of Transportation Vt. Rt. 5A, Westmore

Rockfall Hazards on Vermont Highways

Vt. Route 105, Jay G. Springston, Norwich U. Dept. Geology

Plane failure: slabs dipping toward road.

I-91 South-bound, Weathersfield

Steep potential wedge failure resulting from intersection of the joint sets.

Foliation-parallel joint set with steep dip toward road.

Joint set dipping steeply to right.

G. Springston, Norwich U. Dept. Geology

Fracture-bounded Column

U.S. Rt. 7, Exit 2, Bennington, G. Springston, Norwich U. Dept. Geology

Tension Crack Cc

Ow

Rockfall Hazards on Vermont Highways

TOTAL CUTS = 3,647

A (High Rockfall Potential) = 174 Other

RHRS PRELIMINARY RANKING

4% 2%10%

8%

76%

High A

Significant B+

Elevated B

Moderate B-

Low C